Spero Therapeutics , Inc. shares plunged around 15% after-hours following the company’s announcement, according to which the U.S. Food and Drug Administration (FDA) had completed its review of the New Drug Application (NDA) for tebipenem HBr oral tablets for the treatment of adult patients with complicated urinary tract infection (cUTI) and determined that the NDA could not be approved in its present form.

The agency concluded that the company’s Phase 3 cUTI study of tebipenem HBr (ADAPT-PO) was insufficient to support approval and that additional clinical study would be required, setting a Prescription Drug User Fee Act (PDUFA) target action date of June 27.

The company intends to promptly request a Type A meeting with the FDA in order to gain further insights as to the pathway forward towards a potential regulatory approval for tebipenem HBr.